[cgrums]

Morning All,

I populated my second CK^2III board that I'm going to be using in my work setup along with a pair of new A900s. I took a page out of others design books by putting the toroidal transformer in it's own 'wall-wart' box with an umbilical running up to the box. All parts are those recommended by AMB.

The problem: the measured voltage across either of the secondaries from the transformer are ~20 VAC unloaded. This is just as expected. When I connect one of the secondaries (one only) to the CK^2III board the measured secondary voltage drops to ~8.3 VAC. This is not right.

I only connected one of the secondaries because I'm short on the crimp connectors for the recommended terminal blocks.

I observed the same effect on both AC input points from both secondaries.

The secondary that is unloaded continues to measure ~20 VAC when the other secondary is connected to the board.

Any thoughts? I'm kind of stumped. I'm going to use alligator clips when I get home tonight to connect both secondaries at the same time.

Thanks,
cgrums

 

[tomb]

Actually, 20VAC is a little much for a 15VAC secondary - even unloaded, IMHO. Please tell us what transformer and how you have the leads connected. Then we can compare it with the data sheet for that transformer. It sounds like you have something messed up.

 

[cgrums]

I was wondering about that. It's the Amveco recommended by AMB: TE62033

I'm reasonably sure I've got it wired properly but will have to check that when I get home. As the transformers 'wall-wart' enclosure has been glued together I'm reluctant to open it back up.

FWIW, AMBs operating points pdf lists the voltage source entering the 7815/7915 as '20 Voff' which I interpreted as 20 VAC unloaded. Of course I could be wrong.

Edit: Closer inspection of Amvecos data sheet shows 20.3 VAC as its spec'd unloaded voltage. That should be right. What really weirds me out is the fact that it drops to 8.23 VAC when it's connected to the board thus resulting in a regulated 4.4 VDC at the output.

 

[tomb]

Well, I suppose 20V is possible, but that means toroids are pretty horrible when it comes to internal regulation. As for the connections just to check:

You should have Yellow-to-Red and Black-to-Violet connected on the Primary side. The Secondaries should remain separate, of course. One of your board power leads should be Green and Red, the other Brown and Blue. If you have these connections correct, then perhaps there's a bad short on the board that brings the voltage way down on the secondary when you connect it. If you used a combined plug like Amb's mini-DIN, be sure you didn't get the wires crossed in that arrangement.

 

[cgrums]

Checked the transformer...wired properly.

Inspected the board for any visible shorts...none.

I'm wondering if I can't use the two treads I have lying around to effectively remove the PSU section of the CK^2III.

I'm really hoping there isn't something wrong with the board itself.

Any other ideas?

 

[error401]

Do the regulators get hot? It sounds like there's a short somewhere to me...

 

[cgrums]

quick update: I took the regulators out and everything was fine. I put the 7815 back in and everything was fine, I put the 7915 back in and everything went dead.

Hmm.

I have two extra TREADs lying around so I wired those in to completely remove the CK^2III PS section.

Same problem.

Something tells me their is something in the amplifier section that is shorting the -15V rail to ground.

Stupid question but I'm assuming the logical next step is to look at all the circuit locations that connect directly to the -15V rail and search for solder bridges and shorts, no?